1. Field of the Invention
This invention relates generally to light amplifier tubes, and is concerned more particularly with an image intensifier tube having shielding means for protecting dielectric members during processing and operation of the tube.
2. Discussion of the Prior Art
Generally, an image intensifier tube comprises an evacuated envelope having therein an input screen aligned with a spaced output screen. The input screen, in response to an incident radiational image, emits an equivalent electron image which is electrostatically accelerated to impinge on the output screen with sufficient kinetic energy for producing a corresponding visual image. Consequently, there may be disposed between the input and output screens a coaxial series of spaced grid electrodes which are maintained at suitable electrical potentials for focusing the electron image onto the output screen. Adjacent grid electrodes may be insulatingly secured to one another by interposed dielectric spacers having respective portions attached to the electrodes.
The input screen may include a scintillator layer of fluorescent material disposed on a radiation transmissive substrate which is mounted in the input end portion of the envelope during assembly of the tube. Also, the input screen comprises a photocathode layer of conductive photoemissive materials, which may be vapor deposited on the scintillator layer during processing of the tube. However, it has been found that during deposition of the photocathode layer, vaporous photoemissive materials also may migrate to the dielectric spacers and condense on the surfaces thereof. Consequently, during subsequent operation of the tube, this condensed photoemissive material may provide leakage paths whereby flashover may occur across the dielectric spacers and result in voltage breakdown of the tube.
Therefore, it is necessary and desirable to provide an intensifier tube with shielding means for protecting dielectric members during processing and avoiding voltage breakdown during subsequent operation of the tube.